Along the path to appliance enlightenment I once obtained, by means of money, a 1300W Panasonic "Inverter" Microwave from Sears. (About 5 Years ago). Apparently, the master appliantologists at Panasonic had discovered a way to make a man sized microwave (2.1 cu. ft.) that weighs less than a small child. No more heavy transformer, no expletives required during balancing it end-in with one hand, while plugging it into the hood fan cavity with the other.

Regrettably, I have on two occasions had to disrupt the tranquility of my garden kitchen by making use of those same expletives which I had so painstakingly conserved during installation. (Well...three times, if you count my son’s popcorn-fire incident.)

On the other two occasions, I observed spectacular bright blue flashes coming from outside the cavity, on the bottom of the microwave, even mildly sooting the surface on which the microwave rest. This was followed by that unmistakable smell which I like to think of as “Who let the smoke out of the semiconductors?”

On the first occasion, one year into “quest for fire”, the radiation device in question was under warranty. The friendly Sears service department was more than happy to replace my inverter board, and ding Panasonic for the courtesy. On the second occasion, Sears was ready and willing to relieve me of $135 or so for the same courtesy as the warranty had long since expired.

I declined, since the inside of my microwave was pretty scorched from the popcorn incident two years prior, and since I concluded that this product failure was not a one-time freak accident, but rather a failure one could look forward to on a regular basis.

This is no doubt owing to the fact that the Panasonic appliantologists had clearly not corrected their design inadequacies. (Whether this is due physical inadequacy in other dimensions of their journey, or a simple result of greed on the part of the Panasonic (or should I say Matsushita?) corporation, this is difficult to determine.) Certainly my own designs have always been very reliable, so perhaps there is a correlation between endowment and design quality. Perhaps by ‘extension’, Matsushita Corporation is just the innocent victim of their own winky impaired appliance design community, who is to say?

It is clear to me now however, that for whatever reason, the Master Appliantologists (I choose the second word with some hesitation) who perpetrated this horrible joke on us “innocently trusting in Panasonic quality average-Joe-consumers”, are completely reprobate, and have no intention whatsoever of correcting their design abomination.

Oh no! In fact Panasonic is still enthusiastically perpetrating (or should I say prosecuting?) this hoax here in North America with great zeal. I came to this conclusion just this past week, when in passing I mentioned my experience to a fellow appliantologist whereupon he informed me that he has had the same repair performed twice himself, under Sears Warranty, on his Panasonic Inverter Microwave!!!!! Meanwhile Inverter technology is being gleefully promoted in the Panasonic brochures as though it were a feature and a benefit and not the horrible design failure which we now all know it to be.

As a public service to your honorable readers, I post here, for their benefit, the following caveat:

It is sad to hear your sad experience but glad that we have learnt.
You should be re-classified above " the Master Appliantologists". Hope our Honorable Samurai Appliance Repairman could take the appropriate action as he likes.

I don't have "1300W Panasonic Inverter Microwave" nor any other microwave in the "hood fan cavity". Have only a simple Panasonic microwave oven on the countertop.

I have two queries in mind.
(1) Is "Inverter" technology "wrong" or "bad"?
(2) Is the "hood fan cavity" a good place to house a microwave oven?

I don't have any solid answers but some simple thoughts.

a) With the advent of power electronics, "heavy and large" electro-magnetic devices (transformers, switches, etc) could be replaced by "light and small" power electronic devices (inverter, triac, SCR etc). No one likes to carry a "heavy" note-book PC with a heavy transformer. An aeroplane will not able to take off if heavy transformers are used in place of the inverters.
Electro-magnetic devices are "well-proven" and power electronic devices are "to be well-proven". It takes time and experience for an advanced technolgy to "mature".

b) The hood fan cavity is the worst place in the kitchen where "heat, moiture, oil vapour etc" are concentrated. You are really cooking the "microwave and its inverter" there. Electro-magnetic will have a higher tolerance than power electronic in this condition. Iceman will become "vapourman" there!

1) Can we cut the Panasonic engineers some slack, since they should be recognized for attempting to improve microwave technology by employing modern advances in semiconductor power electronics and, after all, nobody gets it 100% right the first time?

2) Isn't iceman's microwave a special case because it is in a harsh environment?

Regarding the second point:

The Panasonic Microwave in question lived in a special hood-fan cavity that was designed to hold a (very large) microwave. It replaced an even larger Toshiba whose uP power supply went out of regulation rendering it insane but still capable of cooking, just randomly. The oven heat and cooking gases were routed on either side of the microwave cubby hole in special integral ducting in the hood fan unit, and were expelled back into the room 10 inches above the microwave, after passing through two sets of coarse, then fine filters above the microwave oven (one set on each side of the microwave). There was an air gap of approximately one inch on either side of the microwave between the microwave and the ducting and about 5 inches above the microwave. While it is possible that would result in elevated temperatures in the vicinity of the microwave cubby hole, the temperature in that locale was never more than 5 to 7 C above ambient. Also, the microwave was rarely operated at the same time as the stove. The microwave outer surfaces never became contaminated with grease, so I am reasonably confident that this was not a contributor to internal contamination. I have also visually verified this by bringing a special TORX driver with a hole in the center to remove the tamper proof screws home from the office and opening the oven up to survey the damage. Conclusion: no particularly harsh conditions existed in the operating theater of the defective microwave oven.

Regarding the photo, I would love to, but I have recently moved and while the microwave accompanied us, the hood fan unit did not. The hood fan unit was manufactured by Broan in approximately 1977. I have tried to find a picture on the internet, but have not succeeded. I realize now what a unique piece of equipment that was. For example, the fine filters had actual charcoal particles in them.

Regarding the first point:

I applaud Panasonic's attempt at innovation. I am all for miniaturization (except in two specific cases that come to mind) and I am all for advancing the state of the art. However, I also work for a large corporation, and I have worked in a consumer electronics division in that corporation for a period in my career about seven years ago. The name of the game in consumer electronics is cost-reduction, in fact that is the game in every division now. Cost reduction design is where the men are separated from the boys. Any knave can reduce the cost of a circuit, but it takes a master to do so while retaining the same features, and level of quality. One of my colleagues event added functionality on one of our products while cutting the cost to one quarter, when others scoffed and said he was doomed to fail!

More importantly, if the warranty return rate exceeds say 5%, then you know you have a design problem, and responsible companies take action to rectify the problem. Finally, if the design problem is causing fires, that usually gets lot of executive "help" in addition to the regular "support" from product management.

This is the purpose of my post. Do other owners of Panasonic Inverter technology have the same experience as I, or are the four instances of failure I am aware of simply statistical outliers?

I don't know, so at this moment a more reasonable chap might be unbiased. Perhaps it was premature for me to insult the honorable Panasonic design engineers, but in my books, if three-five years elapse and the problem is still not fixed, someone is asleep at the switch, or worse, negligent!

Perhaps I am being a little overly defensive. When I was ten years old some roofers near my house called me a paranoid little weirdo - in Morse code!

I am sorry if I came across a little sardonic. I was trying to be funny, but I would be the first to admit that I have a slightly bent sense of humor.

Seven, I very much enjoyed your commentary on the thermostat problem, and I concluded from it that you are a deep thinker, and someone from whom I can learn much. I still owe the the thermostat bellows photo on that, and I have not forgotton. I am planning to test your theory by taking a before picture then cutting the capillary carefully with a hacksaw to see if it expands further. I will do so as soon as I get my workbench set-up.

Thanks for the warning. I was in a conversation the other day where this reared its head. Did you happen to trace the inverter circuit to see which device let the smoke escape?

Inverter technology is used all over now from audio amplifiers to gas generators. They generally work by converting the AC to DC using a bridge rectifier on the input. Then they use some MOSFETs to chop the DC at a very high frequency to create a square wave. This is then fed to a transformer to step up the voltage as needed to run whatever it is that they need to run.

The weight savings comes because the transformer is operating at a high frequency. A 60 Hz transformer needs to be large to avoid saturation in the iron core. At 60 KHz (or whatever higher frequency is being used) the transformer can shrink greatly while avoiding saturation. Problems sometimes arise because you are dealing with square waves, not sine waves. This is particularly true with compressor motors. You should never run an air conditioner off an inverter/generator that doesn't have a pure sine wave converter on it.

I can see from reading your post that you are very wise in the ways of transformer size reduction, and understand that higher frequencies are a friend of the power designer. Regrettably they are a curse to the rest of us. In fact, I was just suppressing some unpleasant thoughts about conducted emissions on a current project when I came across your excellent tutorial. May I be the first to compliment you on your succinct summary of the essence of inverter design, not to mention the very appropriate warning about not using a square peg in a round hole (pardon my metaphor). I would also add that anything that has low frequency magnetics (e.g. 60Hz transformers) do not like square waves, although you can get away with it on small power cubes if you keep them cool, and if the inverter doesn't choke (current limit) on the unexpected heavy load.

Regarding your question, my best guess is the FETs failed. Typically this is caused by excess heating in the devices, which happens if the snubber circuit fails to do it's job. The snubber fails when either the diode or the cap give up.

This thing that really bugs me about this is the design problem is probably very easy to fix. Something simple like "wrong PIV on the snubber diode" or equally trivial. However, if you don't look at your warranty center return data, you will never know that you have a problem, much less how to fix it.

I was hoping that the provocation directed towards Panasonic (Matsushita) in the invective above would stimulate a sentient response. Perhaps if I was a bit more explicit.

PANASONIC MICROWAVE DESIGN CENTER: I have a smoking example of what is wrong with your product in my garage. Send me your FedEx account number and authorization to ship, and it is yours.

Ahso, I too have just experienced the enlightenment that the inverter board brings. My Kenmore Elite microwave oven brought me to this state after less than 2.5 years of ownership. It welcomed me into a new level of consciousness with the message "F-9."

Unfortunately, the Sears repairman was not familiar with this message, but ah, I had discovered it on a little piece of paper hidden inside the control panel. The fact that he could not interpret "F-9" caused me some concern. But lo, he could call a technical help desk that only Sears Samurais may call and they explained to him the meaning of this message.

My worst fears were realized when he said to me what the little piece of paper had already told me... replace the inverter board. Total cost of the experience, he said, would be $290, which, I believe is more than half the price of my 2-year-old microwave.

I was dismayed, to say the least. This was just yesterday, and I am still adjusting to the out-of-body experience that followed. What must one do?

The Sears Samurai left with $65 and I have 30 days to decide whether I want to contribute $225 more to complete my journey.

The phrase "built-in obsolescence" keeps coming to my consciousness, as does the phrase "rip-off." But I may take another path. With a brother and sister-in-law in the appliance business, I may make a pilgrimage to their doorstep.

That's what I didn't want to do... have it repaired and then see it fry again in a year or two. My brother, who manages an appliance store, recommended I buy a less expensive model (hopefully without the dreaded inverter) since we usually use the microwave just to heat things up and cook simple dishes. That's what we plan to do, when we can afford to replace it.

I have an over the range NN-H264 that was bought in August 2004. Already had to replace a turntable motor on it, under warranty, because the thing made an atrocious grinding noise. Took about 3 months just to get the part.

Now, by the postings below, I'm guessing I have an inverter issue again. I used the thing to thaw 2.5 lbs of food earlier. I had come back and the time was missing from the LED. Reset the time. Went back about 2hrs later or so and put it on for 1min 45 to heat some water and it died completely.

Checked the breaker, good. Checked the outlet with an electric hand mixer, good. Unplugged for a while, plugged back in, nothing.

Given the experience you've had with inverter boards and such, is it worth getting this monster fixed? Given that I paid $278 for 1.5yrs of use, I am trying to weigh as to whether this is even worth the service call. If it's more than a fuse and it's an inverter board issue, I'm guessing I'm out at least $150 for repairs.

In checking websites for this issue, I certainly wished I'd known before my purchase. My last Panasonic lasted 12 years, but that was before the inverter turbo defrost was available.

It still has a 5yr warranty on the tube, but given some of the feedbacks I've been coming across, this probably has nothing to do with the tube.

I actually ended up getting the GE spacemaker 1.8cu ft as a replacement. It has most of the bells and whistles the Panasonic did but I couldn't get one without a handle on the door.

I ended up paying more for this one than I did for the Panasonic, but thus far, it has a quieter operation, still has the defrost features and "quick" buttons, like beverages and such, but the cooking cavity seems bigger. (it probably is since it holds 2 racks, meaning 3 layers of foods)

The scavengers from the garbage company have probably taken away the Panasonic thinking some dumb homeowners threw this thing away b/c they don't know how to fix it. I was told it was probably either the power supply or the inverter board. If it was the power supply, it would have been ok to replace, if it was the inverter board, it was a LOT more expensive. For $60 to have someone come out, then another $50-$150 depending on what the issue was, it wasn't worth investing for a $275 micro.

The forum here helped since I at least knew the inverters were prone parts. Made the replacement decision much easier.